1. Field of the Invention
The present invention relates to a passive electro-optical display cell and, more particularly, to a passive electro-optical display cell comprising a layer of an electro-optical material interposed between two plates, at least one of which is transparent, in which control of the cell is achieved through multiplexing.
2. Description of the Prior Art
U.S. Pat. No. 4,137,524 describes a passive electro-optical display cell comprising a layer of a passive electro-optical material interposed between two plates, at least one of which is transparent, and which further comprises control electrodes and a counter-electrode which are disposed on respective sides of the layer of electro-optical material. A screen electrode is disposed between the control electrodes and the electro-optical material. The screen electrode has an opening facing each of the control electrodes.
If the electro-optical material used is a nematic-phase liquid crystal, polarizing means are disposed on respective sides of the cell, with the directions of polarization thereof being at an angle of 90.degree. relative to each other (crossed polarizers). Alignment layers are also deposited on the internal faces of the two plates in order to induce, in the absence of an electrical field, the helical configuration which is well known in display cells of the twisted nematic type.
In such a cell, the shape and dimensions of the openings in the screen determine the shape and the dimensions of the segments. The segments comprise the zones of the cell which may be made individually visible or invisible with respect to the background of the display and which, by virtue of the various combinations thereof, form different characters, such as numerals, letters or other symbols which are to be displayed.
In the background region of the display, outside the regions corresponding to the above-mentioned openings, the appearance of the cell depends only on the voltage applied between the screen and the counter-electrode. If that voltage is zero, the background of the cell appears light. If the voltage is sufficient for the field that it generates in the liquid crystal to cause alignment of the molecules of the liquid crystal in a direction perpendicular to the plates, the background of the cell appears dark.
At the location of the segments, it is the applied voltage between the control electrodes and the counter-electrode which determines the appearance of the cell. The cell is light in those regions where the applied voltage is zero and dark if the voltage is of sufficient value.
The presence of the screen, therefore, makes it possible to produce a display of data in the form of dark symbols on a light background, that is to say, a positive mode, or in the form of light symbols on a dark background, that is to say, a negative or reverse mode.
In other embodiments, a dichroic coloring agent and, optionally, a chiralic compound is added to the liquid crystal. At least one of the polarizing means may then be omitted. In contrast to the previously discussed case, the cell appears dark in the absence of an electrical field and light when there is such a field present. The mode of operation of this cell is, therefore, precisely the opposite to that described above, but the cell also has the property of permitting a positive or negative display.
In all cases, the presence of the screen also prevents the line of the conducting tracks which connect the electrodes to the terminals of the cell from becoming visible, in spite of the fact that the counter-electrode covers the whole of the rear plate. The problems involved in designing the counter-electrode of a conventional cell and reciprocal alignment of the electrodes and the counter-electrode are therefore eliminated.
A need presently exists for displaying more data than can be displayed by a normal cell. For example, when the cell is intended to be used in an electronic watch which has a large number of time-related or non-time-related functions, it is desirable to be able to display data in the form of numerals indicating the time and letters indicating the day of the week or the name of the month. This may be achieved by using characters which are all formed from a sufficient number of segments, but this is to the detriment of the aesthetic appearance of such characters.
It is sometimes also desirable for the cell to be capable of displaying data of a greatly different nature, such as a map of the world in two hemispheres, to make it possible to locate the position of the time zones in respect of which the time is indicated. This problem may be overcome by the superposition of two separate display cells, but this is at the disadvantage of cost and the thickness of the device.
As soon as the number of numerical, alphabetical or other symbols which can be displayed by a single cell is increased, problems arise in regard to connecting the different control electrodes. In particular, the number of lead-out wires typically increases to a very high level.